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The plasma design point during stationary phase operation of EU-DEMO has been described extensively in the past, and the assumptions employed for its definition have been supported by various investigations. However, a stationary snapshot is by far not sufficient to ensure the actual operability of the plasma scenario. On the con-trary, both normal and off-normal transients have a strong impact on the machine design, providing the most challenging requirements to the actuators. This has to be accounted for in the design from the early phases on. In this paper, the main requirements originating from plasma operation, studied during the Pre-Concept Design Phase, are briefly reviewed. These encompass the so-called planned transients, e.g. plasma current ramps, access and termination of the burn phase or plasma fueling with pellets, but also the mitigation of off-normal events which may lead to a disruption, e.g. the divertor reattachment or an uncontrolled increase of radiation after a sudden W influx from the wall.

Photon-induced reactions on nucleons and light nuclei are measured at the large solid angle spectrometer SAPHIR at ELSA in Bonn.

A high-power sheet beam electron gun for a quasi-optical gyrotron has been developed. The gun produces two beams with rectangular cross sections, with one beam segmented into 11 beamlets. The beamlets are spaced so as to be in line with the maxima of the RF field profile in the resonator of a 120-GHz quasi-optical gyrotron. The maximum current in the segmented beam is nominally 34 A, while the unsegmented beam is 68 A. The gun has been tested, and it has operated essentially as predicted by 2-D and 3-D theory. In particular, the overall beam shape is largely undistorted, and the beamlets are, for most cases, well aligned. >

The effect of doping on the steady-state photoconductivity, σ p , of hydrogenated amorphous silicon (a-Si:H) was explored down to temperatures of 4.2 K. Thermal quenching (TQ) is found in both n-type and p-type samples. The temperature of TQ decreases monotonically as the Fermi level is moved downward through the gap center. Electrons are the photocarriers below TQ and holes above TQ in undoped and p-type a-Si:H. A determination of type of photocarriers for hopping photoconductivity below 40 K yields electrons for n-type and holes for strongly p-type a-Si:H. The localization radii of localized conduction band tail states, a ∼ 13 A, and of valence band tail states, a ∼ 7 A, were determined from the dependence of the photocurrent on temperature and high electric fields. The temperature dependence of σ p of amorphous boron and boron carbon alloys is compared with those of other amorphous semiconductors.

This Paper reviews the design and functionality of the 24MW 170GHz electron cyclotron heating and current drive system being planned for the ITER Tokamak. The sub-systems (power supplies, gyrotrons, transmission lines and launcher antennas) are described based on present day technologies, while on-going R&D provides component and sub-system testing with the possibility of increasing the reliability of the overall EC system. Modifications to the steering ranges of the launching antennas are under investigation that can improve the functional capabilities of the EC system without increasing cost and relaxing the engineering constraints.

The temperature TQ of thermal quenching of the photoconductivity σ p was found to increase monotonically as the Fermi energy is raised through the gap center from p-type to n-type hydrogenated amorphous silicon (a-Si:H). The sign of the majority photocarriers, determined from the magnitude of the drift mobility, changes with increasing T from negative at T TQ for both p- and n-type samples. By using subgap light, hv σ p / eG of electrons in n-type samples is 5 times larger than that of holes in p-type samples for T σ p at low T and in sufficiently strongly doped p-type samples the electron contribution is negligible.

The development of high power gyrotrons (118 GHz, 140 GHz) in continuous wave operation (CW) has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. In this frame, two 140 GHz prototype gyrotrons for CW operation had been constructed and tested at the Forschungszentrum Karlsruhe. According to the results of the prototypes, seven 140 GHz CW gyrotrons were ordered. The first tube was operated at the Forschungszentrum Karlsruhe. A power of 950 kW at efficiency of 43 % (with energy recovery) could be obtained for pulse lengths of 180 s (limited by the available high-voltage power supply). A 30 minute pulse was performed with an output power of 540 kW. During this pulse almost no decrease in performance was found, especially the tube pressure only increased in the range of 10/sup -9/ mbar.

Through a diverse research programme, the Tokamak a Configuration Variable (TCV) addresses physics issues and develops tools for ITER and for the longer term goals of nuclear fusion, relying especially on its extreme plasma shaping and electron cyclotron resonance heating (ECRH) launching flexibility and preparing for an ECRH and NBI power upgrade. Localized edge heating was unexpectedly found to decrease the period and relative energy loss of edge localized modes (ELMs). Successful ELM pacing has been demonstrated by following individual ELM detection with an ECRH power cut before turning the power back up to trigger the next ELM, the duration of the cut determining the ELM period. Negative triangularity was also seen to reduce the ELM energy release. H-mode studies have focused on the L-H threshold dependence on the main ion species and on the divertor leg length. Both L- and H-modes have been explored in the snowflake configuration with emphasis on edge measurements, revealing that the heat flux to the strike points on the secondary separatrix increases as the X-points approach each other, well before they coalesce. In L-mode, a systematic scan of the auxiliary power deposition profile, with no effect on confinement, has ruled it out as the cause of confinement degradation. An ECRH power absorption observer based on transmitted stray radiation was validated for eventual polarization control. A new profile control methodology was introduced, relying on real-time modelling to supplement diagnostic information; the RAPTOR current transport code in particular has been employed for joint control of the internal inductance and central temperature. An internal inductance controller using the ohmic transformer has also been demonstrated. Fundamental investigations of neoclassical tearing mode (NTM) seed island formation by sawtooth crashes and of NTM destabilization in the absence of a sawtooth trigger were carried out. Both stabilizing and destabilizing agents (electron cyclotron current drive on or inside the q = 1 surface, respectively) were used to pace sawtooth oscillations, permitting precise control of their period. Locking of the sawtooth period to a pre-defined ECRH modulation period was also demonstrated. Sawtooth control has permitted nearly failsafe NTM prevention when combined with backup NTM stabilization by ECRH.

Note: Proc. 18th Symposium on Fusion Technology, Karlsruhe, Germany, August 1994, Vol. 1, 489 - 492 (1995) ISBN 0 444 82220 8 Reference CRPP-CONF-1995-042 Record created on 2008-05-13, modified on 2017-05-12

The nonlinear evolution of Langmuir waves excited by the beam-plasma instability is studied experimentally. The emission of electromagnetic waves at the second harmonic of the plasma frequency and the formation of a density depression have been observed. These phenomena are consistent with the predictions of the strong Langmuir turbulence theory.

Reference CRPP-ARTICLE-1985-008View record in Web of Science Record created on 2008-04-16, modified on 2017-05-12

current through the magnetized sample of 20 wt% in the two opposite directions which indicate asymmetry in the resistance of the sample (Fig. 1). Fig. 2 shows the temperature dependence of the reduced resistance, ie the ratio of the resistance at a given temperature (reduced against the critical temperature of the matrix) to the resistance in the normal state, for the sample containing 20 wt% ferromag. netic particles, both before and after magnetization for the both current directions. The estimated maximum errors of the measured values are shown in Fig. 2. As can be seen from Fig. 2, the values of the resistance measured with the current passing through the sample in the same direction as the magnetizating current are larger than the respective resistance values measured with the current passing in the opposite direction. The directional asymmetry of the electrical resistance has been found for aU circularly magnetized samples with the exception of the sample of 5 wt%.